1. Field of the Invention
The present invention relates to an improved sample transfer arm assembly and more specifically to a lost motion clutch mechanism incorporated within the assembly for permitting overrotation of the drive shaft without damage to the assembly when arcuate movement of an arm of the assembly is limited by engagement with a locating stop.
2. Description of the Prior Art
In one type of centrifugal automatic chemical analysis apparatus, a transfer disc having a plurality of spoke like channels formed therein with each channel having at least three wells is utilized in mixing sample and reagent and transferring the mixture to a reaction chamber. Starting from the inner radial end of each channel, there are first and second wells each of which forms either a reagent receiving well or a sample receiving well. The outermost well forms a mixing well. Such transfer disc with, for example, 36 channels, has each of the first and second wells filled with sample and reagent with one channel being filled with water. Then the transfer disc is inserted in a centrifugal chemical analysis apparatus such as the Rotochem.sub.TM sold by American Instrument Company, a division of Baxter Travenol Laboratories, Inc. of Deerfield, Ill. The apparatus is then operated to rotate the transfer disc to cause sample to mix with reagent and then to be ejected from the mixing well by centrifugal force through an outer opening at the end of each channel in the transfer disc and into a reaction chamber or cuvette in an annular ring positioned around and rotatable with the transfer disc.
Light transmitting windows are provided at the top and bottom of each reaction cuvette and the annular ring is positioned to rotate past a light beam located on one side of the annular ring. A photosensitive device located on the other side of the annular ring in the light path of the light beamed from the light source senses the amount of light transmitted to monitor thereby the reaction and, upon each rotation of a reaction chamber 360.degree., the rate of the reaction taking place in each reaction chamber.
In preparing a plurality of samples for analysis in the centrifugal chemical analysis apparatus, the transfer disc is placed on an automatic reagent and sample filling device such as the Rotofill.sub.TM sold by American Instrument Company, a division of Travenol Laboratories, Inc., of Deerfield, Ill. The automatic sample and reagent filling device is provided with a turntable on which the transfer disc is positioned and an annular sample tray supporting a plurality of sample cuvettes in a ring is positioned around the transfer disc and on the turntable. Positioned above the turntable is a stationary reagent arm for dispensing reagent to one or the other of the first and second wells in each channel of the transfer disc.
Additionally, a sample transfer arm is positioned over the turntable and is mounted to and extends radially outwardly from a cylindrical support mounted on a rotatable and reciprocal drive shaft so that the transfer arm can be rotated about one end thereof a predetermined arcuate extent. To insure proper positioning of the transfer arm at a first position over a sample containing cuvette in the same tray, a cylindrical sleeve with a generally rectangularly shaped window through which the arm extends is received over the cylindrical support with one side edge of the window forming a stop which is located and fixed in place to locate the outer end of the transfer arm, when it engages that side edge of the window, over a sample cuvette in the sample tray. Then, a second cylindrical sleeve having a similar generally rectangularly shaped window is positioned over the first sleeve with the window thereof in registry with the first window and with an edge thereof forming a stop to locate the outer end of the transfer arm at the other end of its arcuate path of travel over the innermost well of the transfer disc. Lastly, a third cylindrical sleeve is received over the first two sleeves and has a generally rectangularly shaped window which is in registry with the first two windows. The third sleeve is rotatable between one releasably fixed locating position where an edge of the window therein is radially in line with the locating edge of the window in the second sleeve and a second releasably fixed locating position where the stop forming edge of the window is now positioned to stop movement of the transfer arm with the outer end thereof positioned over the second well of a channel in the transfer disc.
In operation of the automatic reagent and sample filling device, water is placed in the wells of one channel. Then the turntable is indexed a predetermined amount to rotate a first sample cuvette in the sample tray to a position under the outer end of the sample transfer arm. Then the drive shaft, cylindrical support and transfer arm are moved downwardly to bring an aspirating dip tube mounted on the outer end of the transfer arm into the sample cuvette for aspirating a predetermined amount of sample into the dip tube. Then the drive shaft, cylindrical support and transfer arm are moved upwardly and rotated counterclockwise to move the sample arm against the side edge of the window in the second sleeve to locate the outer end of the sample transfer arm over the innermost well, unless, of course, the third sleeve had been moved to locate the edge of the window thereof to stop movement of the transfer arm with the outer end thereof over the second well. In either event, the drive shaft, cylindrical support and transfer arm are lowered to lower the dip tube into the well and the predetermined amount of sample in the dip tube is ejected into the well. Then the sample transfer arm is raised and rotated clockwise to bring the transfer arm back to its first position and the turntable is indexed to position the next sample cuvette under the dip tube at the end of the sample transfer arm to repeat the above sequence of operations. Reagent, of course, is dispensed into the first or second well by dip tubes extending from the reagent arm each time a new channel is positioned under the reagent arm.
In the operation of the automatic reagent and sample filling device, exact control of the rotational movement of the drive shaft is not readily obtainable. As a result, stress and a bending moment are placed on the transfer arm when it engages one of the stop forming side edges of one of the windows when the drive shaft rotates more than the predetermined arcuate extent. Also a torque is placed on the drive shaft and the cylindrical support for the transfer arm. These stresses, bending moments and torque forces have resulted in damage to the device and failure of the device to operate properly.
As will be described in greater detail hereinafter, the present invention provides a solution to this problem of bending moments and torque forces being placed on the sample transfer arm assembly by providing a lost motion clutch assembly in a drive arrangement between the cylindrical support and the drive shaft.
The lost motion clutch assembly permits overrotation of the drive shaft beyond the predetermined arcuate extent without damage to the transfer arm assembly when arcuate movement of the transfer arm is limited by engagement thereof with one of the locating stops defined by a side edge of one of the windows in one of the sleeves.